Method for forming insulating coatings on metal surfaces



IUD-( UKUOD [\Ll bill-u- Oct. 26, 1965 R. BRODT ETAL 3 31 METHOD FOR FORMING INSULATING COATINGS ON METAL SURFACES Filed Feb. 16, 1962 METALLIC SURFACE COAT WITH AQUEOUS SOLUTION CONTAINING 50-500 G/L ALKALI METAL SILICATE (OPTIONALLY ALSO METALLIC OXIDE OR HEXAVALENT CHROMIUM COMPOUND) HEAT AT 300-l200C. TO FORM ANNEALED COATING COAT WITH AQUEOUS ACID PHOSPHATE OR ACID HEXAVALENT CHROMIUM SOLUTION HEAT AT 300-I200C. FOR AT LEAST l MINUTE METALLIC SURFACE WITH INSULATING COATING RUDOLF BRODT EBERHARD BRONDER l/VVE/VTOPS M MIQM ATTORNEY United States Patent METHOD FOR FORIVHNG INS G COATINGS 0N METAL S ACES The present invention relates to a method for forming adherent coatings on metal surfaces which are useful to provide electrical insulation, heat resistance, equity; resistance as a basejgr paint, varnishes and the like, and may used as an aid in the deformation of metals. More particularly, this invention relates to a method of forming adherent coatings on metal surfaces by applying aqueous solutions, dispersions or suspensions to the metal surface and heating the coated metal surface at an elevated temperature to effect interreaction and adhesion of the coating to the surface.

A number of processes have been proposed, in the past, for the purpose of forming electrically insulating coatings on metal surfaces. Such processes include two basically different types. The first of these merely mechanically applies a coating to the surface and the cotating is dried on the surface, for example, the application of paste to paper surfaces, or the application of varnish coatings or the like. The second type involves the formation of a chemical coating on the surface, that is, one which is obtained through a reaction between the components of the applied material and the surface to effect adherence and coating formation. Thus, for instance, phosphate coatings have been applied for the purpose of producing heat insulating coatings by the use of alkaline earth metal phosphates and annealing the coating on a surface. Such coatings provide heat resistance up to about 1000 C. in a properly protective gas atmosphere. It is also known that the heat resistance of coatings formed from aqueous acidic metal phosphate solutions such as iron phosphate, zinc phosphate and manganese phosph alei l o weuhan ObQnedJQmJheJse-OI .htli egil kalines earth metal phosphates.

Itis a primary object of this invention to provide a process for coating metal surfaces with an adherent coating which is capable of protecting the metal surface against corrosion at temperatures as high as about Another object of this invention is to provide an improved process for coating metal surfaces with an electrically insulating coating which renders the coated metal suitable for use in a variety of electrical applications such as transformer cores, magnet cores, and the like.

Another object of this invention is to provide an improved method for coating metal surfaces with a coating which is sufliciently adherent to permit deformation of the metal into the desired final form and to retain sufiicient coating on the surface to be an effective electrical insulator.

This invention is related to the invention disclosed in United States Serial 173,830, filed February 16, 1962, which application has inventors and assignee common to this application.

This invention represents a modification of the process described in US. application Serial 173,830, filed February 16, 1962, in that it may employ the compositions and procedures described therein as the first step in the process of this invention.

In accordance with this invention it has been found that improved tempgatgmresistanmfilli llegtwce is imparted to aqueous 1 -'uced coatings on metal surfaces when the annealed water glass coating is subjected to a sec ggtr eagnent Whichiorms an additional overlying or single butmodified coating on the metal surface. For this second step treatment, it has been found to be satisfactory to coat the annealed water glass coated metal surface with an aqueous hexavalent or dispersion or with an a I on, suspension or dispersion, which second coating is then dried and annealed at an elevated temperature to secure uniformity and adherence of the total coating.

Phgsphate solutions which are suitable for the second step application in the process of this invention include aqueous solutions of calcium, barium, magnesium, strontium, beryllium and aluminum phos hates and mixtures thereof, particularly the mono-phosphate solutions of these metals. For this purpose the compositions may include from about 70 grams P0 per liter up to about 400 grams PO, per liter, but somewhat better results are obtained if the P0 is maintained above about grams per liter. A preferred concentration is in the range of grams PO, per liter to about 340 grams P0 per liter- Such aqueous phosphate solutions should include a sufiicient quantity of free phosphoric acid to keep the phosphate salt in solution and concurrently avoid the formation of a precipitate. The preferred quantity of free phosphoric acid is that amount which corresponds to the equilibrium condition which maintains the phosphate salt in solution and avoids precipitation, and since the equilibrum solubility of the phosphates increases as phosphoric acid is added, the particular quantity which is necessary will depend upon the P0 concentration which is selected.

Thickness of the phosphate coating which is to be formed may be partially controlled by varying the concentration of the phosphate in the solution. I f des1red, however, the thickness may be partially controlled by the use of filler aterials sughmmnnltfimmghlyji persed gL a, glumirmntertiarycalcium pho? e. Such fillers are preferably added to the phosphate solution in the form of fine particles to thus form a fine dispersion. The compositions may also include other heat resistant oxides such as titanium dioxide, chromic oxide, and zirconium oxide. otherfilmzybe formed during the annealing process apgl may porated in teso gignjnmtheiorm of a Mistlmnonndsuch, for example, as boric acid ammonium borate, the polysilicates, the polyphosphates, and specifically the ammtminm, es and polyphosphates. The phosphate solutions may be modified to contain normally decomposable phosphates such as the phosphates of ammonia or derivatives thereof including urea phosphates, aniline phosphates or the organic esters of phosphoric acid such as methyl, ethyl, propyl and butyl phosphate esters. Pyridine phosphates and the alkyl substituted ammonium phosphate containing less than 7 carbon atoms may also be employed. In the compositions which include both an alkaline earth metal phosphate such as calcium phosphate and a thermally decomposable phosphate, it has been found to be desirable to maintain the P0 concentration at not less than about 100 grams per liter and in this case the primary calcium phosphate may vary within the range of about 100 to 250 grams per liter, calculated as Ca(H PO -1H O. The thermally decomposable phosphate may vary relative to the calcium phosphate or its equivalent within the range of ratios of primary NH, to primary Ca or other alkaline earth metal, inclu 6 to 1 and l to 1. ln su'ch compositions the ratio of free PO, to the total PG, content, expressed as the ratio of free P 0 to total P 0 should be within the range of 1 to 7.7 and 1 to 2.9.

When the second step employs hexavalent chromium containing materials, it is preferable to utilize water soluble hexavalent chromium-containing compounds such as the alkali metal chromates and dichromates. It is however, satisfactory to employ water insoluble hexavalent chromium containing compounds such as the zinc dichromate, zinc tetroxy chromate and the like. In either event, a satisfactory concentration of such hexavalent chromium compound is one which is equivalent to about 50 to 100 grams per liter of CrO In accordance with the multi-step process of this invention, it has been found that this is satisfactory to employ as a first step solely an aqueous water glass composition. For this purpose, the water glass concentration may vary within the range of about 50 to about 500 grams per liter, depending upon the thickness of the first layer coating which is desired. As above indicated, the initial layer of the multi-layer coating of this invention may be formed satisfactorily by using any of the alkaline earth metal oxide, or hexavalent chromium modified aqueous water glass compositions described and claimed in copending application Serial No. 173,830. As set forth therein, in such compositions the hexavalent chromium-containing compounds which may be used include zinc tetroxychromate, zinc dichromate, and the like, as well as the alkali metal chromates and dichromates and the alkaline earth metal chromates. When the second step in the process of this invention involves the use of a hexavalent chromium containing solution or suspension, it is preferable to use a water glass concentration in the range of 300 to 500 grams per liter. However, when an aqueous water glass solution containing one of the alkaline earth metal oxides or aluminum oxides is employed for forming the first layer coating, it is preferred to use a concentration of water glass within the range of about 80 to about 250 grams per liter.

In summary, the aqueausanatmgltggggmi e first layer may include about 50 to a on grams per liter of at least one oxide selected from the group consisting of calcium oxide, magnesium oxide, barium oxide, strontium oxide, aluminum oxide and a hexavalent chromium containing co amount equivalent to about 50 to about 100 grams per liter of CrO The compositions may include from about .25 to about grams per liter of an oxide of a multi-valent metal selected from the group consisting of titanium, vanadium, chrgmium, manganese, iron, cobalt and nickel.

ter the aqueous water glass coating is applied in the quantity sufiicient to produce a coating thickness in the range of about 2 to about microns and preferably about 3 to about 10 microns, the water content is carefully evaporated from the coating to initially fix that coating on the surface. Subsequently, the coating is then subjected to an elevated temperature curing in the range of about 300 to about 1200 C. and for the purposes of this invention it is preferred to employ a curing temperature in the range of about 500 to about 900 C.

After the first layer of the coating is annealed and cooled, the coating is ready for the application of the selected phosphate or hexavalent chromium containing solution to form the overlying or modified two layer coating. The second layer may be applied by using the same procedure as that employed in the formation of the first layer, namely, by dipping, spraying or rolling, preferably with grooved rollers. The quantity of material which is applied is controlled to produce a relatively thin coating having a total thickness not exceeding about 20 microns. The phosphate or hexavalent chromium-containing second layer of the coating is then cured at a temperature in the range of 300 C. to 1200 C. and preferably in the range of 500 C. to 900 C. Somewhat better results are obtained when the minimum temperature of 600 C. is employed and at this temperature a good coating is formed in about 5 minutes. As the temperature is increased toward the upper limit, the required time to effect a satisfactory cure decreases so that at about 1200 C. satisfactory cure is achieved in about 1 to 2 minutes.

The following examples will serve to illustrate this invention in somewhat greater detail.

Example I An aqueous solution was prepared using commercial sodium silicate water glass having a density of 1.385 to 1.41 at a concentration of 50 grams per liter of the water glass.

Cold rolled electro sheet stock containing 3% silicon and carbon of less than 0.01% was then coated with the aqueous water glass solution by using grooved rollers. The water was evaporated fromthe coating by using infra-red bulb heaters and then the coated metal parts were positioned in a nitrogen atmosphere furnace and maintained therein at 900 C. for 5 minutes and withdrawn. An inspection of the coating showed that it had a thickness of 3-4 microns.

The rupturing voltage of the coating was established by placing an electrode with a load of grams per square centimeter on the coated surface, the electrical circuit including a transformer capable of providing alternating voltage of varying intensity and the bare sheet metal to thus complete the circuit. The rupturing voltage was established by slowly increasing the alternating voltage from the transformer from zero voltage slowly until a reak-down of the coating took place, and the voltage at the break-down was recorded as the rupturing voltage. Each coating was tested at a plurality of locations over its entire surface and the reported value for the rupturing voltage is the average of this plurality of separate location rupturing voltage readings.

Using this test on the water glass coated surface, the rupturing voltage was found to be between 10 and volts.

A number of sheets coated with the annealed water glass coating, as above described, were coated with an aqueous composition containing grams per liter Example ll Electro sheets of the type specified in Example I were coated with an aqueous water glass solution containing 100 grams per liter water glass dried and annealed for 5 minutes at 900 C. in a nitrogen atmosphere furnace. The coatings had an average thickness of 34 microns and a rupturing voltage of 10-90 volts, when determined by the procedure outlined in detail in Example I.

A plurality of these sheets were coated with the phosphate solution of Example I, dried and annealed for l-2 minutes at 550 C. in an air atmosphere furnace. The total coating thickness was about 4-5 microns and the rupturing voltage was found to be in the range of 200 to 230 volts.

Example III Electro sheets of the type identified in Example I were rubber roller coated with an aqueous water glass com position containing 200 grams per liter of water glass. After drying and annealing for 5 minutes at 900 C. in a nitrogen atmosphere furnace, these sheets were found to be coated with a coating having a thickness in the range of 4-5 microns. When tested for rupturing voltage by using the test described in Example I, the coatings were found to have an average rupturing voltage in the range of to 200 volts.

A plurality of the coated sheets were coated with the phosphate composition identified in Example I and, after drying, were annealed in an air atmosphere furnace at 550 C. for 1-2 minutes. After annealing, the coatings had a thickness of about 5 to 6 microns and the rupturing voltage was determined to be in the range of 200 to 230 volts.

Example IV Electro sheets of the type identified in Example I were rubber roller coated with an aqueous water glass solution containing 400 grams per liter of water glass. After preliminary drying and annealing for 5 minutes at 900 C. in a nitrogen atmosphere furnace, the average coating thickness was found to be from 4-5 microns and the rupturing voltage from 150-200 volts.

A plurality of the coated sheets were given a second coating with the phosphate composition identified in Example I and, after preliminarily drying and annealing at 550 C. foug minutes in an air atmosphere furnace, the coating t was ound to be about 5-6 microns. The average rupturing voltage was found to be in the range of 200 to 230 volts.

Example V An aqueous water glass solution was prepared, using the commercial water glas identified in Example I, containing 50 grams per liter water glass and 60 grams per liter of finely-powdered magnesium oxide.

Electro sheet of the type specified in Example I was rubber roller coated with this water glass composition, preliminarily dried and annealed for 4 hours at 1000 C. in a pure nitrogen atmosphere furnace. After cooling in the nitrogen atmosphere furnace, the coatings were checked for rupturing voltage and the average was found to be in the range of 80-150 volts.

A number of the sheets were given a second phosphate coating with the composition identified in Example I. After preliminarily drying and annealing in air at 550 C. for 1-2 minutes, the coating thickness was found to average about 5 microns and the rupturing voltage was found to be 250 volts and higher.

Another plurality of the water glass coated panels were provided with a phosphate coating from the same solution and, after preliminarily drying, were annealed for 4 hours at 840 C. in a 95% nitrogen-5% hydrogen atmosphere furnace and the total layer thickness was still about 5 microns. The rupturing voltage for these coatings was found to be in the range of 220-240 volts.

Example VI An aqueous water glass solution was prepared containing 100 grams per liter of water glass and 60 grams per liter of finely-powered magnesium oxide.

Electro sheet of the type identified in Example I was coated with this water glass solution, preliminarily dried and annealed for 4 hours at 1000 C. in a furnace containing an atmosphere of 80% nitrogen-20% hydrogen. After cooling, the sheet was coated with the same phosphate coating solution identified in Example I, dried and annealed for 1-2 minutes in air at 550 C. Inspection revealed that the coatings had an average thickness of about 5 microns and a rupturing voltage which was 250 volts and higher.

Example VII Electro sheet of the type identified in Example I were coated with an aqueous solution containing 300 grams per liter of sodium silicate water glass. After preliminarily drying, the coatings were annealed in a pure nitro gen atmosphere furnace for 10 minutes at 900 C. After cooling, the surfaces were coated with a second layer from an aqueous solution containing 200 grams per liter of ammonium dichromate and 0.5 gram per liter of a non-ionic wetting agent of the hydrogenated tallow amide type, in which the nitrogen thereof is substituted with 6 mols of ethylene oxide, commercially available under the name Emulgator Ethomide HT/ 25. After drying, the coating was annealed in a pure nitrogen atmosphere furnace for 10 minutes at 900 C. and the rupturing voltage was found to be in the range of 140-240 volts.

A number of sheets were coated with the 300 grams per liter sodium water glass solution and after annealing as above described, were given a second coating from a composition containing 200 grams per liter of ammonium dichromate, 10 grams per liter of H SiF and 0.5 gram per liter of the non-ionic emulsifier identified above. After preliminarily drying and annealing for 10 minutes at 900 C. in a pure nitrogen atmosphere furnace, the coatings were found to have substantially the same thickness, 3-5 microns, as those obtained from the treatment not including the H SiF ingredient and a rupturing voltage in the range of 140-200 volts.

Example VIII Electro sheet of the type identified in Example I were given a first coat of a solution containing 500 grams per liter of sodium silicate water glass, dried and annealed for 10 minutes at 900 C. After cooling, the sheets were given a second coating from an aqueous solution containing grams per liter of CrO After drying and annealing in a pure nitrogen atmosphere furnace at 900 C., the coatings were found to have an average rupturing voltage in the range of 150-250 volts.

Another series of sheets were coated with the same water glass solution as the first coating and given a second coating from an aqueous solution containing 100 grams per liter of CrO and 10 grams per liter of H SiF After drying and annealing for 10 minutes in a pure nitrogen atmosphere furnace at 900 C., the coatings were found to have an average rupturing voltage of 150-200 volts.

Example IX An electro sheet was given a first coating from an aqueous solution containing 100 grams per liter of sodium silicate water glass. After drying and annealing in a pure nitrogen atmosphere furnace for 10 minutes at 900 C. and cooling, the sheets were given a second coating from a solution containing 200 grams per liter ammonium dichromate, 50 grams per liter powdered magniesium oxide, 30 grams per liter acetic acid and 0.5 gram per liter of the non-ionic wetting agent identified above in Example VII. After drying and annealing in a pure nitrogen atmosphere for 10 minutes at 900 C. the coatings were found to have an average rupturing voltage in the range of 100-200 volts.

Example X Electro sheet was given a first coating from an aqueous solution containing 100 grams per liter of sodium silicate water glass and 50 grams per liter of powdered magnesium oxide. After drying, and annealing in a pure nitrogen atmosphere furnace for 10 minutes at 900 C. the coated sheets were given a second coating from an aqueous solution containing 200 grams per liter of ammonium dichromate and .5 gram per liter of a non-ionic emulsifier as identified above in Example VII. The rupturing voltage was established by using the procedure detailed in Example I and found to be within the range of -220 volts.

Using similar application techniques, drying and annealing conditions and testing for rupturing voltage in the same manner described in Example I, highly satisfactory coatings and rupturing voltages of 200 volts and above were obtained from the following compositions:

(a) A first coating from a solution containing 100 grams per liter sodium silicate, 50 grams per liter magnesium oxide and 3.9 grams per liter FeOH A second coating from an aqueous solution containing 200 grams per liter ammonium dichromate and .5 non-ionic emulsifier identified in Example VII.

(b) A first coating from an aqueous solution containing 300 grams per liter of sodium silicate water glass.

A second coating from an aqueous phosphate composition containing 250 grams per liter Ca(H PO 1 H 0,

130 grams per liter ammonium dihydrogen phosphate,

NH H PO 108 grams per liter free P (added as H PO (c) A first coating from an aqueous solution containing A second coating was formed from an aqueous dispersion containing 110 grams per liter barium phosphate, BaH (PO 62 grams per liter NH H PO 52 gram sper liter free P 0 grams per liter TiO 10 grams per liter bentoni te clay.

What is claimed is:

1. A process for forming insulating coatings on metallic surfaces which comprises the steps of applying to said surface an aqueous composition comprising as its essential ingredient about 50 to about 500 grams/ liter of a silicate of an alkali metal selected from the group consisting of sodium and potassium, subjecting the said coated surface to an elevated temperature in the range of about 300 C. to about 1200 C. to form an annealed coating on said surface, contacting said coating with an aqueous composition selected from the group consisting of aqueous acidic phosphates and aqueous hexavalent chromium-containing compositions, and subjecting the coated surface to a temperature in the range of 300 C. to 1200 C., to thus form an adherent coating on said surface.

2. A process for forming insulating coatings on metallic surfaces which comprises the steps of applying to said surface an aqueous composition comprising as its essential ingredients about 50 to about 500 grams/liter of a silicate of an alkali metal selected from the group consisting of sod-rum and potassium, about 50 to about 100 grams/liter of at least one metallic oxide selected from the group consisting of magnesium oxide, calcium oxide, barium oxide, strontium oxide, alumingmjxid-fi a hexavalent chromium-containing compound in an amount equivalent to about 50 to 100 grams/liter of CrO and zero to about 10 grams/liter of an oxide of a multivalent metal selected from the group consisting of titanium, v chromium, manganese, iron, cobalt and nickel, su-b' said co to a temperature within the range of about 300 C. to about 1200 C. for at least about one minute, cooling said surface and applying to said cooled surface a composition selected from the group consisting of an aqueous acidic phosphate composition containing from about 70 to about 400 grams/liter of PO, and an aqueous acidic hexavalent chromium composition containing said hexavalent chromium compound in an amount equivalent to about 50 to about 100 grams/liter of CrO drying said coating and subjecting said dried coating to a temperature in the range of about 300 C. to about 1200 C. for at least one minute.

3. A process for forming insulating coatings on metallic surfaces which comprises the steps of applying to said surface an aqueous composition comprising as its essential ingredient about 50 to about 500 grams/liter of a silicate of an alkali metal selected from the group consisting of sodium and potassium, subjecting the said coated surface to a temperature in the range of 500 C. to 900 C. to form an annealed coating on said surface, contacting said coating with an aqueous composition selected from the group consisting of aqueous acidic phosphates and aqueous hexavalent chromium-containing compositions, and subjecting the coated surface to a temperature in the range of 500 C. to 900 C., to thus form an adherent coating on said surface.

4. A process for forming insulating coatings on metallic surfaces which comprises the steps of applying to said surface an aqueous composition comprising as its essential ingredient about 50 to about 500 grams/liter of a silicate of an alkali metal selected from the group consisting of sodium and potassium, about 50 to about 100 grams/liter of at least one metallic oxide selected from the group consisting of magnesium oxide, calcium oxide, barium oxide, strontium oxide, aluminum oxide a hexavalent chromium-containing compound in an amount equivalent to about 50 to 100 grams/liter of CrO and zero to about 10 grams/liter of an oxide of a multivalent metal selected from the group consisting of titanium, vanaidum, chromium, manganese, iron, cobalt and nickel, subjecting said coated surface to a temperature within the range of about 500 C. to about 900 C. for at least one minute, cooling said surface and applying to said cooled surface a composition selected from the group consisting of an aqueous acidic phosphate composition containing from about 70 to about 400 grams/ liter of P0 and an aqueous acidic hexavalent chromium composition containing said hexavalent chromium compound in an amount equivalent to about 50 to about 100 grams/liter of CrO drying said coating and subjecting said dried coating to a temperature in the range of about 500 C. to about 900 C. for at least one minute.

5. A process in accordance with claim 1, wherein said second coating is formed from an aqueous acidic phosphate solution.

6. A process in accordance with claim 1 wherein said second coating is formed from an aqueous hexavalent chromium compound-containing composition.

7. A process for forming insulating coatings on metallic surfaces which comprises the steps of applying to said surface an aqueous composition comprising as its essential ingredients about to about 250 grams/liter of a silicate of an alkali metal selected from the group consisting of sodium and potassium, about 50 to about grams/liter of at least one metallic oxide selected from the group consisting of magnesium oxide, calcium oxide, barium oxide, strontium oxide, and aluminum oxide, and a hexavalent chromium-containing compound in an amount equivalent to about 50 to 100 grams/liter of Cr0 and zero to about 10 grams/liter of an oxide of a multi-valent metal selected from the group consisting of titanium, vanadium, chromium, manganese, iron, cobalt and nickel, subjecting said coated surface to a temperature within the range of about 300 C. to about 1200 C. for at least about one minute, cooling said surface and applying to said cooled surface a composition selected from the group consisting of an aqueous acidic phosphate composition containing from about 70 to about 400 grams/liter of P0 and an aqueous acidic hexavalent chromium composition containing said hexavalent chromium compound in an amount equivalent to about 50 to about 100 grams/liter of CrO drying said coating and subjecting said dried coating to a temperature in the range of about 300 C. to about 1200 C. for at least one minute.

8. A process for forming insulating coatings on metallic surfaces which comprises the steps of applying to said surface an aqueous composition comprising as its essential ingredients about 80 to about 250 grams/liter of a silicate of an alkali metal selected from the group consisting of sodium and potassium, about 50 to about 100 grams/liter of at least one metallic oxide selected from the group consisting of magnesium oxide, calcium oxide, barium oxide, strontium oxide, and aluminum oxide and a hexavalent chromium-containing compound in an amount equivalent to about 50 to 100 grams/liter of CrO and up to about 10 grams per liter of an oxide of a multivalent metal selected from the group consisting of titanium, vanadium, chromium, manganese, iron, cobalt and nickel, subjecting said coated surface to a temperature within the range of about 300 C. to about 1200 C. for at least about one minute, cooling said surface and applying to said cooled surface a composition selected from the group consisting of an aqueous acidic phosphate composition containing from about 70 to about 400 grams/liter of P0 and an aqueous acidic hexavalent chromium composition containing said hexavalent chromium compound in an amount equivalent to about 50 to about 100 grams/liter of CrO drying said coating and subjecting said dried coating to a temperature in the range of about 300 C. to about 1200" C. for at least one minute.

9. A process for forming insulating coatings on metallic surfaces which comprises the steps of applying to said surface an aqueous composition comprising as its essential ingredients about 300-500 grams/liter of a silicate of an alkali metal selected from the group consisting of sodium and potassium, subjecting said coated surface to a temperature within the range of about 300 C. to about 1200 C. for at least about one minute, cooling said surface and applying to said cooled surface an aqueous acidic hexavalent chromium composition containing said hexavalent chromium compound in an amount equivalent to about 50 to about 100 grams/liter of CrO drying said coating and subjecting said dried coating to a temperature in the range of about 300 C. to about 1200 C. for at least one minute.

10. A process for forming insulating coatings on metallic surfaces which comprises the steps of applying to said surface an aqueous composition comprising as its essential ingredients about 300-500 grams/liter of a silicate of an alkali metal selected from the group consisting of sodium and potassium, subjecting said coated surface to a temperature within the range of about 300 C. to about 1200 C. for at least about one minute, cooling said surface and applying to said cooled surface a solution containing as the essential coating producing ingredients between about 70 grams P0 per liter and 400 grams P0 per liter of an alkaline earth metal phosphate, an amount of phosphoric acid sufiicient to prevent the formation of a precipitate from the said phosphate in said solution, and a thermally decomposable phosphate containing ammonium, said thermally decomposable phosphate being present in an amount such that the ratio of primary N11,, to primary alkaline earth metal in said phosphate is in the range of 0.56 to 1 and 10 1 to 1, and thereafter subjecting said surface to a temperature between about 300 C. and 1200 C. for a period of time sufficient to yield a firmly adherent coating on said surface.

11. A process for forming insulating coatings on metallic surfaces which comprises the steps of applying to said surface an aqueous composition comprising as its essential ingredients about 50-500 grams/liter of a silicate of an alkali metal selected from the group consisting of sodium and potassium, subjecting said coated surface to a temperature within the range of about 300 C. to about 1200 C. for at least about one minute, cooling said surface and applying to said cooled surface a solution containing as the essential coating producing ingredients between about 70 grams P0 per liter and 400 grams P0 per liter of an alkaline earth metal phosphate, an amount of phosphoric acid suflicient to prevent the formation of a precipitate from the said phosphate in said solution, and a thermally decomposable phosphate selected from the group consisting of ammonium phosphate, urea phosphate and aniline phosphate, said thermally decomposable phosphate being present in an amount such that the ratio of primary NH to primary alkaline earth metal in said phosphate is in the range of 0.56 to 1 and 1 to 1, and thereafter subjecting said surface to a temperature between about 300 C. and 1200 C. for a period of time sufficient to yield a firmly adherent coating on said surface.

References Cited by the Examiner UNITED STATES PATENTS 944,957 12/09 Eberhard 106-74 1,924,311 8/33 Frey 117-127 X 1,946,146 2/34 Kiefer et al 117-129 X 1,951,039 3/34 Scharschu 117-127 X 1,990,075 2/35 Horak 106-38.27 2,218,058 10/40 Stalhane. 2,418,608 4/47 Thompson et al. 2,438,013 3/48 Tanner. 2,493,934 1/50 Waring 148-62 2,529,206 11/50 Winslow et al. 106-74 X 6,265,232 1/54 Neish 117-127 X 2,909,454 10/59 Neish 117-127 X 2,989,418 6/61 Harbaugh 106-74 X 3,030,217 4/62 Chantler et al 106-84 X FOREIGN PATENTS 162,980 4/ 37 Australia.

453,226 9/36 Great Britain.

464,967 4/37 Great Britain.

875,972 8/61 Great Britain.

WILLIAM D. MARTIN, Primary Examiner.

' MURRAY KATZ, Examiner.

UNITED STATESPATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,214,302 October 26, 1965 Rudolf Brodt et a1.

It is hereby certified that error appears in the above numbered patent requiring correction and that the eaid Letters Patent should read as corrected below.

Column 1, line 28, for "cotating" read coating column 3, line 11, for "this" read it column 5, line 26, for "glas" read glass column 6, line 45, for "magniesium" read magnesium column 7, line 27, for "gram sper" read grams per same column 7, line 53, and column 8, line 19, after "aluminum oxide", each occurrence, insert and column 8, line 24 for "vanaidum" read vanadium line 51, strike out "and"; line 52, for "oxide," read oxide column 9, line 3, strike out "and".

Signed and sealed this 4th day of April 1967.

Attcat:

ERNEST W. SWIDER EDWARD J. BRENNER Atteeting Officer Commissioner of Patente 

1. A PROCESS FOR FORMING INSULATING COATINGS ON METALLIC SURFACES WHICH COMPRISES THE STEPS OF APPLYING TO SAID SURFACE AN AQUEOUS COMPOSITION COMPRISING AS ITS ESSENTIAL INGREDIENT ABOUT 50 TO ABOUT 500 GRAMS/LITER OF A SILICATE OF AN ALKALI METAL SELECTED FROM THE GROUP CONSISTING OF SODIUM AND POTASSIUM, SUBJECTING THE SAID COATED SURFACE TO AN ELEVATED TEMPERATURE IN THE RANGE OF ABOUT 300*C. TO ABOUT 1200*C. TO FORM AN ANNEALED COATING ON SAID SURFACE, CONTACTING SAID COATING WITH AN AQUEOUS COMPOSITION SELECTED FROM THE GROUP CONSISTING OF AQUEOUS ACIDICPHOSPHATES AND AQUEOUS HEXAVALENT CHROMIUM-CONTAINING COMPOSITIONS, AND SUBJECTING THE COATED SURFACE TO A TEMPERATURE IN THE RANGE OF 300*C. TO 1200*C., TO THUS FORM AN ADHERENT COATING ON SAID SURFACE. 